maint: avoid 'const fooPtr' in cpu files

maint: avoid 'const fooPtr' in cpu files

'const fooPtr' is the same as 'foo * const' (the pointer won't
change, but it's contents can).  But in general, if an interface
is trying to be const-correct, it should be using 'const foo *'
(the pointer is to data that can't be changed).

Fix up offenders in src/cpu.

* src/cpu/cpu.h (cpuArchDecode, cpuArchEncode, cpuArchUpdate)
(cpuArchHasFeature, cpuDecode, cpuEncode, cpuUpdate)
(cpuHasFeature): Use intended type.
* src/conf/cpu_conf.h (virCPUDefCopyModel, virCPUDefCopy):
Likewise.
(virCPUDefParseXML): Drop const.
* src/cpu/cpu.c (cpuDecode, cpuEncode, cpuUpdate, cpuHasFeature):
Fix fallout.
* src/cpu/cpu_x86.c (x86ModelFromCPU, x86ModelSubtractCPU)
(x86DecodeCPUData, x86EncodePolicy, x86Encode, x86UpdateCustom)
(x86UpdateHostModel, x86Update, x86HasFeature): Likewise.
* src/cpu/cpu_s390.c (s390Decode): Likewise.
* src/cpu/cpu_arm.c (ArmDecode): Likewise.
* src/cpu/cpu_powerpc.c (ppcModelFromCPU, ppcCompute, ppcDecode)
(ppcUpdate): Likewise.
* src/conf/cpu_conf.c (virCPUDefCopyModel, virCPUDefCopy)
(virCPUDefParseXML): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>